Electrical slip rings are well known in the art for communicating electrical signals between structural members which are rotatable relative to each other.
A useful method of manufacturing large diameter electrical slip ring bases, commonly referred to as wrapped ring technology, is disclosed in Bowman et al. U.S. Pat. No. 5,054,189, the contents of which are hereby incorporated by reference. As described therein, a slip ring assembly may include an annular base member having a plurality of conductive rings surrounding an outer circumferential face thereof, and a series of electrically conductive brushes arranged on another structural member, wherein one of the annular base member and the structural member is relatively rotatable about the other. In such a configuration, the conductive rings are contacted by the conductive brushes to form a plurality of electrical connections between the two structural members.
As disclosed in the prior patent, a method of manufacture of the slip ring base may include forming a series of annular grooves in the circumferential outer face of an annular base, for receiving rings of electrically conductive material. Linear conductive material is cut to lengths, forming strips each of which substantially corresponds to the circumference of the outer face of the annular base, to form a plurality of electrically conductive rings. First ends of respective rings are anchored at respective points on respective grooves, and the respective rings are pressed into engagement within the respective grooves (e.g., by progressively exerting a rolling pressure along the lengths of the rings around the circumference of the outer face). The other ends of the respective rings are secured to the slip ring base. In one application of the wrapped ring technology, the conductive strips are pre plated with a conductive material, such as silver.
One advantage of the wrapped ring technology over known solid ring technologies is that, in the wrapped ring technology, inexpensive strip-form material can be used, instead of the more expensive solid rings.
However, while slip ring assemblies manufactured by the wrapped ring technology described above are useful, such assemblies suffer from the following deficiencies.
In the known art, the accumulation of manufacturing tolerances results in an inevitable gap between the ends of the conductive slip rings.
A small gap between the ring ends is, indeed, generally desirable to assure that the ring ends do not contact during the rolling operation and prevent the ring from properly sitting in the groove. However, the gap resulting from the known manufacturing process is a physical discontinuity having technical consequences on durability of the brushes, on the quality of electrical connection between the conductive rings and the brushes, and on transmission of signals therebetween.
A problem arising from the existence of the gap is the accumulation therein of wear debris, whether from the brushes, rings, or other materials. Once sufficient wear debris accumulates in the gap between the ring ends, such debris can cause an increase in electrical resistance between the brush and the slip ring. Increased contact resistance may occur when the debris is dislodged onto the ring contact surface, for example. Increased contact resistance may introduce electrical noise into signals transmitted between the slip rings and the brushes, or may attenuate signal transmission therebetween.
In one embodiment of a wrapped slip ring described at FIG. 11 in Hirao et al. U.S. Pat. No. 5,224,138, beginning and end portions of a metal tape 264 forming a slip ring are coupled by solder 272. While such an approach may appear to overcome the above described technical problems, in fact the solution creates an additional technical problem of solder smearing.
Solder used in the manner shown in the prior art will smear onto the adjacent ring surface and, as solder is not a precious metal, will oxidize with undesirable characteristics. An accumulation of the smeared oxides will cause increased electrical noise, significantly degrading the ability of the ring to function with low voltage, low current signals. While accumulation of oxide on a slip ring supplying power to CT scanner X-ray tubes may be acceptable, such accumulation is unacceptable for slip rings operating at low level signal voltages.
Thus, use of solder between ring ends is an unacceptable solution which replaces one noise problem in signal transmission by another, similar, problem.
Moreover, solder will wear at a faster rate than the silver. Accordingly, after extended operation, a rut will develop in the solder.
The potential for these or other possible technical problems is one of the chief drawbacks to the wrapped ring technology relative to solid ring technologies. However, as noted above, the wrapped ring technology has an economic advantage over solid ring technologies. This economic advantage becomes more pronounced as the diameter of the device increases.
There is thus a need in the prior art for an improved method of manufacture of slip ring assemblies which maintains the economic advantage of wrapped ring technologies over solid ring technologies, but which overcomes the known deficiencies of known wrapped ring technologies.